The present invention relates to heat exchangers and to electro-discharge machining processes. The present invention also relates to microchannel heat exchangers having a high aspect ratio and methods of making such heat exchangers.
Heat transfer and fluid flow in the microchannels of microchannel heat exchangers have wide practical applications in highly specialized fields such as aerospace, bioengineering, microfabricated fluidic systems, and microelectronics. Lately, microchannel heat exchangers have also been used by the automotive air conditioning industry. The advantage of the microchannel heat exchanger lies in its high heat transfer coefficient and significant potential in decreasing the weight and size of heat exchangers.
Compared with channels of normal size, microchannels have many heat transfer advantages. Since microchannels have an increased heat transfer surface area and a large surface-to-volume ratio, they provide a much higher heat transfer. The surface-to-volume ratio can be expressed as the aspect ratio, which is the ratio of the microchannel depth to the width. The higher the aspect ratio, the greater the surface-to-volume ratio of microchannel heat exchangers. This feature allows microchannel heat exchangers to become compact and lightweight. In addition, microchannel heat exchangers can support high heat flux with small temperature gradients. However, microchannels also have weaknesses, such as large pressure drop, high cost of manufacture, dirt clogging, and flow mal-distribution, especially for two-phase flows. These weaknesses become more prevalent as the aspect ratio and the density of microchannels, and thus the efficiency, of microchannel heat exchangers increases.
For example, U.S. Patent Application Publication No. 2004/0031592 describes a microchannel heat exchanger having a maximum of only 21 fins per inch, the fins being the walls separating discrete microchannels. The aspect ratio of the microchannels of the patent application 2004/0031592 is less than 10. Moreover, the microchannel heat exchanger is made from a dense steel alloy which allows for stronger fins and thus more microchannels per inch. However, as well as becoming more compact, it is desirable to have microchannel heat exchangers that are made of lighter materials.
In another example, U.S. Patent Application Publication No. 2004/0099712 describes microchannel heat exchangers and microreactors having about 25 fins per inch and an aspect ratio of less than 1. They are also made of heavy, dense material such as stainless steel. These microchannel heat exchangers and microreactors would not provide optimal cooling because of the number of fins and low aspect ratio.
As can be seen, there is a need for compact microchannel heat exchangers having a high density of fins per inch as well as a high aspect ratio. It also would be desirable if such compact microchannel heat exchangers were made of a lighter, less dense material than steel alloys. There is also a need for a relatively simple and low-cost manufacturing method for such compact microchannel heat exchangers.